1. Field of the Invention
The present invention relates to the field of packaging of electrical devices. More particularly, the present invention is a method and apparatus for packaging an electrical device.
2. Related Art
Ball grid array packages are commonly used for connecting semiconductor devices to a printed circuit board. Ball grid array packages include a substrate on which a conductive pattern is printed. Typically the conductive traces extend across the full length of the top surface of the substrate. The conductive pattern includes bonding pads that connect to conductive traces. The conductive traces electrically connect to bonding pads located on the bottom of the substrate.
Solder mask is then deposited, masked and etched such that the layer of solder mask covers the entire top surface of the substrate except for the bonding pads. In some prior art processes, a hard protective layer is deposited over the bonding pads so that there is a sufficiently hard surface for wire bonding.
The electrical device to be packaged (e.g., a semiconductor device) is placed over the substrate and a wire bonding process is performed. The wire bonding process electrically connects contacts on the semiconductor device with corresponding bonding pads on the surface of the substrate. A cover is then formed over the top of the structure that covers the semiconductor device and the bonding pads. The substrate is then trimmed. This is usually done by cutting off the edges of the substrate on all four sides.
Conductive balls, typically referred to as solder balls, are formed over the bottom surface of the substrate. The solder balls electrically connect to the bonding pads on the bottom of the substrate, and are thereby electrically connected to the semiconductor device.
Ball grid array packages are usually disposed over an array of contact pads printed on a printed circuit board and heat is applied such that the solder balls adhere to the array of contact pads. Once the solder cools, the ball grid array package is mechanically and electrically coupled to the printed circuit board.
The use of ball grid array packages is quite effective for mechanically and electrically connecting semiconductor devices to a printed circuit board. However, the use of ball grid array packages makes testing or xe2x80x9cdebuggingxe2x80x9d of the semiconductor device difficult. More particularly, because the connections between the ball grid array package and the circuitry of the circuit board are under the ball grid array substrate, they cannot be reached for the purpose of testing.
Some prior art printed circuit boards include test pins that are electrically connected to the solder balls that extend from the printed circuit board. However, at higher frequencies, these pins act as antennas, causing signal degradation (e.g. reflections, impedance mismatches, etc.) This is particularly true with respect to high performance data communication signals (e.g. data communication signals over 5 gigaHertz).
Often, the circuits of the semiconductor device are tested by connecting a testing probe (e.g., an oscilloscope or voltage meter) to leads of other devices that are electrically connected to the ball grid array device. However, this technique requires that there be other devices that are not ball grid array devices that include exposed leads. At high frequencies, devices that have exposed leads cannot be used because of the effects of signal degradation. For example, at frequencies above 5 gHz packages that have leads cannot be used as they have unacceptably high signal degradation.
What is needed is a method and apparatus that allows for testing of the circuits of a device that is packaged using a ball grid array package. In addition, a method and apparatus is needed that meets the above need and that is effective at high frequencies. Moreover, a method and apparatus is needed that meets the above needs and that does not contribute to signal degradation.
A method and apparatus is disclosed that allows for testing of the circuits of a device that is packaged using a ball grid array package. In addition, a method and apparatus is disclosed that meets the above need and that is effective at high frequencies. Moreover, a method and apparatus are disclosed that meets the above needs and that does not contribute to signal degradation.
In one embodiment of the present invention, a method for forming a ball grid array package is disclosed for containing a semiconductor device. First, an interconnect structure is formed on a substrate. The interconnect structure electrically connects the device to be housed in the ball grid array package to the solder balls of the ball grid array. In the present embodiment the interconnect structure includes bonding pads formed on both the top of the substrate and on the bottom of the substrate. The interconnect structure also includes traces and interconnects that electrically couple the bonding pads on the top of the substrate to the bonding pads on the bottom of the substrate.
Contact pads are formed over the top surface of the substrate. These contact pads electrically connect to the interconnect structure. In the present embodiment, the contact pads are formed during the previous step of forming an interconnect structure. More particularly, the contact pads are formed during deposition and etch steps that form the bonding pads and the traces on the top surface of the substrate.
A non-conducting layer is formed over the substrate that includes openings that overlie the contact pads and openings that overlie the bonding pads. In the present embodiment, the non-conducting layer is solder mask material.
The ball grid array is then completed using conventional process steps. In the present embodiment, the ball grid array is completed by forming a housing over the top of the substrate, trimming the substrate and forming solder balls on the bottom surface of the substrate. Thereby, a ball grid array package is formed having solder balls that are electrically coupled to the electrical device housed within the ball grid array package.
The ball grid array package can then be attached to a circuit board. In the present embodiment, the ball grid array package is attached to the circuit board by heating the solder balls such that the solder balls adhere to corresponding bonding pads located on the circuit board.
The contact pads of the ball grid array package of the present invention are exposed through the solder mask. Therefore, these contact pads are easily accessible as they are exposed on the top of the ball grid array package. Thus, even when the ball grid array is attached to a circuit board, connection with the circuits of the enclosed electrical device can be obtained.
Testing can be easily accomplished while the ball grid array package of the present invention is attached to the circuit board via contact with the exposed contact pads. More particularly, an electrical testing device can touch the contact pads for testing the enclosed electrical device.
The exposed contact pads could also be used for coupling to an overlying device or devices. This can be accomplished by disposing a device over the ball grid array package and by making electrical contact via the exposed contact pads. In addition, the exposed contact pads could be used for electrically coupling to devices that do not overlie the ball grid array package.
Because the ball grid array package of the present invention allows for easy testing via the exposed contact pads in the top of ball grid array package, there is no need to form test pins that extend from the printed circuit board as is required in some prior art applications. Therefore, there are no pins that act as antennas, and no corresponding signal degradation (e.g. reflections, impedance mismatches, etc.) as occurs with prior art devices.
In addition, because the ball grid array package of the present invention allows for easy testing via the exposed contact pads in the top of ball grid array package, there is no need for connecting a testing probe (e.g., an oscilloscope or voltage meter) to leads of other devices (that are electrically connected to the ball grid array) for testing the circuits of the electrical device disposed in the ball grid array package. Therefore, there is no need to include devices with leads in a circuit board design for the purpose of testing an electrical device housed in a ball grid array package as is required in some prior art applications.
Moreover, because the ball grid array package of the present invention allows for easy testing via the exposed contact pads in the top of ball grid array package, circuit boards can be easily designed for high performance data communication. More particularly, ball grid array packages can be included in the circuit board design and prior art features that provide for testing can be eliminated (e.g., test pins that extend from the printed circuit board and devices that include accessible leads), producing a circuit board design that can provide high performance data communication (e.g. data communication signals over 5 gigaHertz (gHz)).
Additionally, the methods of the present invention do not require any additional process steps as compared to prior art ball grid array manufacturing methods. More particularly, contact pads are formed during the conventional step of forming bonding pads and traces on the top of the substrate. Openings that extend above the contact pads are formed in the solder mask during the conventional step of forming openings to expose the bonding pads. Therefore, the methods and apparatus of the present invention provide for connectivity to the enclosed electronic device while not requiring additional process steps as compared to prior art processes. Therefore, the ball grid array housing of the present invention is easily and inexpensively manufactured.
Accordingly, the method and apparatus of the present invention allows for testing of the circuits of a device that is packaged using a ball grid array package. In addition, the method and apparatus of the present invention is effective at high frequencies. Moreover the method and apparatus of the present invention do not contribute to signal degradation.